X-Linked Genetic Intellectual Disability #AtoZChallenge

Hi everyone. I can’t remember whether, in my post on the genetics of intellectual disability, I mentioned the difference between autosomal and X-linked genetic causes of intellectual disability. Autosomally genetic conditions are those that are present on one of the autosomes, or non-sex chromosomes, the chromosomes 1 till 22. These present equally commonly in those with two X chromosomes (typically assigned female at birth) as in those with an X and a Y chromosome (typically assigned male). X-linked conditions, on the other hand, present more frequently in either of those groups depending on whether they are recessive or dominant.

An example of an X-linked recessive intellectual disability syndrome is Christianson Syndrome. This affects primarily people assigned male at birth because of their XY chromosomes. After all, recessive means that, if a person has an unaffected copy of the X chromosome, that will be dominant and the person (usually assigned female, XX chromosomes) will not have the condition.

At my former care home, there was a man with Christianson Syndrome. He was in his late fifties, which is really old for someone with the syndrome. He, like everyone with the condition, has an intellectual disability. He also has ataxia, although he at least when I still lived there could still walk, unlike most adults with this syndrome. According to the staff, a lot of things made sense now that they knew that he had this syndrome, even his obsession with electronics.

An example of an X-linked dominant intellectual disability syndrome is Rett Syndrome. This only affects people with two X chromosomes (usually assigned female at birth). Fetuses with XY chromosomes and a Rett Syndrome mutation on their X chromosome, will usually be miscarried.

At my old care home, there was another client with Rett Syndrome. I did not know her well though. Rett Syndrome children develop typically until at least six months of age and then regress. They will develop autistic-like behaviors, although social interaction may develop later on. Another typical feature of Rett Syndrome is the loss of hand function. People with Rett Syndrome will usually have involuntary, repetitive hand movements. I do know the client at my old care home loved to play with sensory toys, but I do not know whether she could purposefully manipulate them.

Another syndrome I need to mention is Fragile X Syndrome. This is an X-linked recessive condition, but it does affect some people with XX chromosomes, albeit more mildly. There is also a so-called “premutation”, which is associated with some Fragile X-linked problems later on but not the full syndrome. This “premutation” is differently inherited depending on the sex of the carrier and may lead to a full mutation (Fragile X Syndrome) in their children.